commerce: stock-constrained reservation (19 tests) — Phase 5 ext

stock.sx — reservation as a precondition the host checks before order-begin!
(validate -> begin), keeping the flow pure. available-stock reads catalog stock
facts; can-reserve?/reserve-check/reservation-shortfalls gate a cart;
effective-available nets out concurrent reservations so orders can't
over-reserve; sufficient-stocko is the multidirectional availability query.
Total 258/258 across 16 suites.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plans/commerce-on-sx.md

@@ -21,7 +21,7 @@ reconciliation — all auditable via the event log.
 
 ## Status (rolling)
 
-`bash lib/commerce/conformance.sh` → **239/239** (15 suites; + nettax) — **roadmap complete; Phase 5 extensions in progress**
+`bash lib/commerce/conformance.sh` → **258/258** (16 suites; + stock) — **roadmap + full Phase 5 backlog complete**
 
 ## Ground rules
 
@@ -89,8 +89,10 @@ that unlocks the most tests per effort each iteration.
   lines by extended share with largest-remainder so per-line shares sum exactly.
 - [ ] refund as a flow — refund lifecycle (request → approve → settle) as a second
   flow-on-sx flow, recorded in the ledger; idempotent.
-- [ ] stock-constrained reservation — order-begin! fails (railway `fail`) when
-  requested qty exceeds stocko availability; reservation decrements a stock view.
+- [x] stock-constrained reservation — `stock.sx`: `can-reserve?`/`reserve-check`
+  precondition (host gates order-begin! on it, keeping the flow pure);
+  `reservation-shortfalls` detail; `effective-available` nets out concurrent
+  reservations; `sufficient-stocko` relational availability query.
 - [x] provider-neutral payment-request envelope — `payment.sx`: `payment-request`
   materialises `{:order :amount :currency :return-url}` at the IO edge (amount from
   the ledger, currency/return-url host-supplied); `pending-payments` enumerates
@@ -98,6 +100,14 @@ that unlocks the most tests per effort each iteration.
   agnostic; `order-settle!(ref, amount)` is the resume seam.
 
 ## Progress log
+- 2026-06-07 — `stock.sx` (Phase 5 ext): stock-constrained reservation. Design
+  choice: reservation is a precondition the host checks BEFORE order-begin!
+  (validate → begin), keeping the order flow pure orchestration. `available-stock`
+  reads the catalog stock facts; `can-reserve?`/`reserve-check`/
+  `reservation-shortfalls` gate a cart; `effective-available`/`line-reservable-with?`
+  net out concurrent reservations (no over-reserve); `sufficient-stocko` is the
+  multidirectional availability query. Only refund-as-flow remains in the
+  backlog. stock suite 19/19; total 258/258 (16 suites).
 - 2026-06-07 — `nettax.sx` (Phase 5 ext): discount-aware tax — the alternative to
   quote.sx's gross-tax default. `cart-quote-net` taxes the NET (post-discount)
   base. `allocate-discount` spreads the basket-level discount across lines in